The invention relates to a process for patterns observing moire patterns of surfaces to be tested by application of the moire method using phase shifting wherein object gratings to be observed are pictured on auxiliary gratings and the moire patterns generated thereby are detected, stored and/or processed by computer, wherein for one observation at least three phase-shifted moire patterns are respectively evaluated. Furthermore, the invention is related to a device for observing moire patterns of surfaces to be tested by application of the moire method using phase shifting, optionally comprising a grating projection arrangement, an object grating of the surface to be tested, an observation objective, an auxiliary grating, a camera arrangement, a memory and/or computer.
The process and the method of the first mentioned type are not restricted to the projection moire method, which is provided for out of plane measurements. Moreover, they can generally be applied to the observation, measuring and documentation of gratings and of their changings to be evaluated by applying the moire method, and therefore they are also provided for the observation and measurement of in-plane deformations, in the case of which gratings are fixedly applied on or in the surface of the object to be tested and thus deformation measurements can be made in the plane. Therefore, object grating here is to be understood being the grating on or in the object as well the projected grating.
By using a process and a device of the first mentioned type surfaces and their deformation are observed, wherein the surface shape and its deformation respectively are taken up and documented. The field of application extends from microscopic observation (e.g. in the field of material testing, biology or medicine) to the observation of surfaces having m.sup.2 size (e.g. in the automobile industry). The observation of surfaces and of their deformation can be made in different ways. Current processes of surfacelike observation are apart from the different moire methods the photogrammetry, the holography, and with restrictions the motography. These methods are applied partially in competition, partially complementary to one another.
In U.S. Pat. No. 4,641,972 a process for determining the surface profile of an object is described, wherein a light beam having a sinusoidally changing intensity pattern is directed to the object, wherein the phase of the sinusoidal pattern is modulated. A deformed grating image of the object is received by means of a detector arrangement for a number of different modulated phases of the impinging light beam. Thus, an object and a reference phase are obtained for the object and a reference plane point-by-point. The height of each point of the object surface is then determined in relation to the reference plane on basis of the phase differences.
The projection moire method is particularly simple and economical for observing surfaces and their deformation. The measuring principle involved is the following: A line grating is projected in an inclined manner at a defined angle onto the surface to be measured, wherein the brightness distribution of the grating generally corresponds to a trigonometric function. The surface is observed by a camera or the like at another angle, the observer or the camera respectively usually being present in the same vertical distance from the surface. Two observations or images respectively are superposed, which have a reference to different surface shapes and in this manner a moire pattern is generated. Thus, a relative measurement is performed using two observations made successively concerning time. Provided determined marginary conditions are kept, the single orders of the moire pattern correspond to equidistant contour lines. Studies of geometric factors affecting the use concerning the projection moire method are described by G. Wutzke in "On geometric factors affectinq the use in moire topography", Materialpru. 20 (1978), No. 9, pages 338 to 342.
In a conventional arrangement disclosed in German published application 35 27 074, operating with application of the projection moire method, provided for determining the surface shape of objects for the use in a stereomicroscope, in particular for the examination and operations of eyes, a projection and a reference grating with optical means for shifting the image of the projected grating and being arranged sequentially to the reference grating are used. In the European published application 0 262 089 a further device for measuring the surface of an object is described, wherein the moire fringe pattern is generated by combining two bar gratings and stripe images deformed at the object surface are taken up by a video camera and are evaluated by an electronic circuit. The bar gratings are shifted with relation to each other, whereby the phase of the fringe pattern can be changed by measurable amounts.
For the evaluation it is often sufficient to use only the moire pattern reduced to the lines of maximum brightness or maximum blackening. However, it is advantageous to apply the phase-shift method instead of such a line-thinning process of the moire pattern. When using the phase-shift method there are required at least three single pick-ups for each of both modulo-2-pi images or phase portraits, which are respectively allocated to a certain surface shape, and for the evaluation a computer is required. Thus, the reference grating of two superposed grating images is respectively observed in its original or basis position, and in two positions shifted thereto, which usually are shifted by 120.degree. in phase so that three intensity distributions are taken up for each object point. Because of the shifting the three pick-ups are taken successively in time. Correspondingly the projection arrangement comprises a suitable device for enabling the phase-shifted grating projection, i.e. the phase shifting The phase-shift method is distinguished by a simple computer processing, since contrary to the line-thinning process the counting direction of the fringes is recognized and in practice no interactive operation is required. Furthermore, resolution is possible into fractions of a moire order. The degree of resolution is herewith dependent on the precision of the shifting and of the quality of the observed sin.sup.2 brightness gradient of the grating.
An application of the phase-shift method is e.g described in B. Breuckmann and W. Thieme "A computer supported holographic system for the use in industry", VDI Report, 552 (1985), pages 27 to 36. A further process and system for the observation of surfaces is known from U.S. Pat. No. 4,212,073, wherein a sinusoidal grating is shifted in three steps by a quarter period of the grating respectively, the intensity of the radiation from the surface is detected and stored for each step. By the use of simple arithmetic operations point heights are determined from these stored values. The conventional phase shift method has the disadvantage that the shape of the surface must not be changed or be changed only in an unessential matter during the time of taking up. Therefore, the phase-shift method could only be applied for static tests or, if really necessary, for the observation of very slow changes. When using this process, dynamic measurements, in particular in the short-time field with dynamic, quickly developing changements of the surface form structures are not possible.
For increasing the sensitivity in case of the projected fringe method, i.e. for increasing the resolution with increasing distance of the contour lines from each other, the projection angle has to be large and the grating scaling has to be as small as possible. A restriction herein results from the digitizing of the image data caused by the computer, which results in a restriction of the spatial frequency. By the provision of an additional auxiliary grating having a suitable slightly different scaling (mismatch) the line density can be increased by one to two orders and thereby the resolution can be increased considerably. The grating projection to be registered is pictured onto this auxiliary grating, with an essentially coarser moire pattern being generated having a line density sufficient for the requirements of digitizing. Accordingly, an auxiliary storing of the grating projection to be registered is made by means of a moire pattern. Since both of the pick-ups being superposed are pictured on the same auxiliary grating, the influence of the latter on the auxiliarily generated pattern of the modulo-2-pi images is cancelled after suposition of two such pick-ups. Corresponding computer supported optical measurement processes are described in "Application of optical processes have a maximum resolution in the field of surface testing and of 3-D measurement technique" by B. Breuckmann and P. Lebeck, VDI Report 679 (1988), pages 71 to 76.